This invention relates to a system and method for reducing and/or detecting magnetic field interference, and in particular, to a system and method for protecting a circuit in a high strength magnetic field.
Magnetic resonance imaging (MRI) is used for non-invasive precision diagnosis of various parts of a body (e.g., soft tissue) without the use of x-rays. An MRI scanner includes a large and very strong magnetic field in which the patient is positioned. A radio antenna is used to transmit an RF signal into the body. If the RF signal is of the correct frequency, protons in the body temporarily align and emit an RF signal weaker than the RF signal transmitted by the antenna. These returning signals are converted into pictures by a computer attached to the scanner.
The strength of the RF signal output from the body is so weak that an ambient RF signal of the same frequency (e.g., a small radio station signal) would wash out the returning signals in an unprotected environment. Therefore, MRI scanners are installed in an MRI suite, and are surrounded by a radio frequency or Faraday shield to protect the MRI scanners from outside RF interference. However, most electronic equipment emits some RF signal at the frequencies of concern. Computers in particular emit an RF signal stronger than the signal coming from the body. Therefore, equipment in an MRI suite must be modified so that it does not emit a radio signal that interferes with the body""s emitted signal.
In the presence of an external magnetic field, protons in the body align with the magnetic field and precess at a specific frequency. A high static magnetic field is required to provide the magnetic environment in which the nuclei will emit an RF signal strong enough to be read. The so-called magnetic resonance is created when these protons are energized by RF waves of the same frequency at which the protons precess. In medical imaging, the field strengths used are typically 1.5 tesla (T) which is about 30 thousand times the strength of the earth""s magnetic field. This same field is strong enough to pull heavy-duty floor buffers and mop buckets into the bore of the magnet, pull stretchers across the room and turn steel oxygen bottles into flying projectiles. Deaths have occurred from trauma as a result of these effects. Smaller objects (e.g., pagers, bobbie pins and pens) have been known to be pulled off the person carrying them in the magnetic suite. The attraction of ferromagnetic objects by the magnet is the most obvious hazard of the high static magnetic field created by an MRI scanner.
MRI suites must be thoroughly surveyed since all environmental iron is paramagnetic and therefore subject to magnetic flux. It is sometimes necessary to reroute pipes and electrical wiring and remove all stationary environmental iron (e.g., structural steel, floor decking, concrete reinforcing rods) from the suite.
There are three issues for ancillary equipment operating in an MRI suite. These issues are: 1) that the attractive effect of the magnetic fields on the ancillary equipment present no danger; 2) that the ancillary equipment functions properly within the magnetic field; and 3) that the ancillary equipment has no intrinsic effects on the quality of the image produced.
Some of the more significant challenges to patient monitoring in an MRI suite are presented by the two large RF coils which surround the patient. Generally, the outer coil transmits the RF while the inner coil receives the RF emitted from the patient. As noted above, the MRI suite must be shielded from outside RF interference which may affect RF reception. The MRI suite can be RF shielded by lining the walls and windows with continuous sheets or screens, typically of copper. Monitors and cables must be shielded to prevent these devices from introducing RF into the room. For example, cables can be wrapped with a thin layer of aluminum foil or woven copper wire and small copper boxes can be used to house electrical equipment.
The effect of the magnetic field in the MRI suite on equipment depends on the strength of the magnet, the proximity of the equipment to the magnet and, the amount of ferromagnetic material present and the details of the circuitry used in the equipment. The magnetic field decreases as an object is moved away from the magnet bore. It is typically recommended that ferromagnetic materials be kept several feet beyond the point where the magnetic field falls to under 50 gauss.
In general, replacing the paramagnetic components in electronic equipment with non-magnetic stainless steel, brass, aluminum or plastic enables its placement within the MRI suite. However, even with the reduced ferrous load, much of the equipment""s smaller, more delicate instrumentation is still ferromagnetic and subject to the magnetic field. Accordingly, the functioning and accuracy of equipment within the suite may still be affected. For example, any piece of equipment utilizing transformers or inductors can malfunction or be damaged in a high strength magnetic field.
MRI magnetic fields will cause transformers or inductor cores to be saturated. Saturation of the transformer or inductor core reduces the core""s permeability (inductance) and allows excessive currents which can burn out the transformer or inductor. Accordingly, when possible, patient monitors are generally kept outside the MRI suite, and an external power source is used.
Open magnet MRIs have been recently designed so that intricate neuro-surgical procedures can be performed within an MRI suite, with the patient and surgeon inside the magnet. The very nature of these procedures increases the need for precise and accurate patient monitoring (e.g., invasive, beat and respiratory monitors) in close proximity to the magnet bore where the greatest magnetic field is present.
Anesthesia machines are being specially designed for use in the MRI suite. Most of the machine""s ferromagnetic components are replaced with brass, aluminum, non-magnetic stainless steel and plastic to minimize attractive forces. In particular, the frame, chassis and drawers are fabricated from aluminum. In addition, many small components, including fasteners and springs, are formed of either aluminum or non-magnetic stainless steel.
Both circle and re-breathing anesthesia systems can be used for ventilation in the MRI suite. Flow sensors are critical to these anesthesia systems for measuring respiratory flow rate. Ultrasonic flow sensors are fast becoming the flow meter of choice in flows where precision, reliability, and maintainability are important. Time of flight (transit-time) ultrasonic flow sensors measure the difference in time travel between pulses sent with and against the fluid flow. The measurement is based on the principle that it takes longer for the ultrasound to propagate upstream than it does downstream.
Whenever a flow is present, there is a difference in the time of flight between the ultrasonic pulses transmitted upstream and downstream in the respiratory flow path. This difference is used to compute the flow velocity which is then factored with the cross-sectional area of the flow path to yield flow volume. Preferably the two transducers are piezoceramic and are arranged in the path of a gas flow to send alternate sound waves towards each other.
The piezoelectric transducers are typically coupled to transmitter and receiver circuitry by two matching standard iron core wire wound transformers. The transformers are sensitive to saturation from the magnetic field of the MRI. That is, the magnetic field of the MRI can cause the transformer""s core to be saturated.
As the permeability of the core decreases, the energy transfer efficiency of the transformer decreases and the impedances of the transformer windings decrease. Both of these changes impact the operation of the transformers that are driving and/or receiving energy from the piezoelectric transducers and therefore affect the operation of the ultrasonic flow sensor. Saturation of the transformer core prevents the production of inductive voltage and excessive currents can burn out a power transformer.
Degradation of a magnetic core is generally gradual up to the saturation flux level of the core. When this level is reached, the core goes into saturation and the permeability of the core approaches the permeability of air. For iron core structures, this can mean a significant change in core permeability. This large change in core permeability causes the gas flow transducer to go from accurate to very inaccurate over a small incremental change in the external magnetic field. In summary, saturation of the transformer core prevents the production of inductive voltage, and excessive currents can burn out the power transformer and adversely affect operation of the sensor.
To insure that the gas flow transducers are not used in a situation where the external magnetic field could cause the problems stated above, it would be beneficial to provide a shield and method of shielding the transformers. It would also be beneficial to provide an approach for measuring the magnetic field strength around the transformers to indicate the presence of a critical external magnetic field.
Briefly stated, preferred embodiments of this invention provide shielding for a circuit in a magnetic field, protect two or more transformers from a condition of saturation in a magnetic field, and provide an indication of a condition where a circuit is being subjected to a magnetic field that would render the circuit inaccurate.
According to a preferred embodiment of the invention, an exemplary assembly for protecting transformers in a magnetic suite includes the transformers transferring electrical energy from an energy source to a processing circuit, and a magnetic shield enclosing the transformers such that the magnetic shield prevents ambient magnetic flux from interfering with the transformer.
According to another preferred embodiment of the invention, an exemplary method for protecting two or more transformers in a magnetic suite is described. The transformers transfer electrical energy from an energy source to a processing circuit. The method includes enclosing the two transformers in a magnetic shield, the magnetic shield preventing ambient magnetic flux from interfering with the transformers, and enclosing the transformers in a radio frequency (RF) shield housing, the RF shield housing preventing RF signals from entering and exiting the housing. The method may also include mounting a magnetic sensor, preferably positioned between the first and second transformers within the magnetic shield, the sensor measuring the magnetic flux magnitude within the magnetic shield. The magnetic sensor may be coupled to a warning circuit for indicating when the magnetic flux magnitude within the magnetic shield exceeds a predetermined threshold. In addition, the method may also include securing the magnetic shield to an anesthesia device, the anesthesia device arranged for operation in an MRI suite proximate an MRI scanner.
According to another preferred embodiment of the invention, a housing protects two or more transformers from saturation via ambient magnetic flux, the transformers transferring electrical energy from an energy source to a processing circuit. The housing encloses the transformers and includes first and second shielding sections. The first shielding section partially encloses the transformer and includes a first shielding layer formed of a material (e.g., iron) that is permeable to the ambient magnetic flux. The second shielding also partially encloses the transformer and includes a second shielding layer formed of a material (e.g., iron) that is permeable to the ambient magnetic flux. The first and second shielding sections are arranged to fit together to at least substantially completely enclose the transformer and shunt the magnetic flux around the transformer. This embodiment of the invention may also include a sensor mounted within the housing to measure the magnetic flux affecting the transformer, and a warning circuit for determining the magnetic flux and indicating when the magnetic flux exceeds a predetermined threshold. Preferably, the processing circuit includes a flow sensor that measures the respiratory flow rate of a person in an MRI suite.